1. Field of the Invnetion
The present invention relates to the processes for synthesizing branched polymers. More specifically, the invention relates to a method for synthesizing graft polymers and, more specifically, those polymers having a dendritic architecture.
2. Description of the Prior Art
Synthetic polymers can take one of two general forms: linear or branched. Linear polymers are composed of the polymer backbone and pendent side groups inherent to the individual repeating units. Branched polymers have discrete units which emanate from the polymer either from the backbone or from the pendent groups extending from the individual repeating units. The branches have the same general chemical constitution as the polymer backbone. The simplest branched polymers, sometimes referred to as comb branched polymers, typically consist of a linear backbone which bears one or more essentially linear pendent side chains. Dendritic polymers are created by adding sub-branches to the branches extending from the main backbone. Dendritic polymers can be subdivided into 3 main categories: dendrimers, hyperbranched polymers and arborescent (or dendrigraft) polymers. Dendrimers are mainly obtained by strictly controlled branching reactions relying on a series of protection-coupling-deprotection reaction cycles involving low molecular weight monomers. Hyperbranched polymers are obtained from one-pot random branching reactions of polyfunctional monomers, resulting in a branched structure that is not as well defined as for dendrimers. Arborescent (or dendrigraft) polymers are obtained by successive grafting reactions of polymeric side chains on a polymer backbone.
Many methods have been developed for the synthesis of dendritic polymers which can be separated into four general categories of reactions. The first category involves a one-step process in which hyperbranched polymers are produced during polymerization as described in U.S. Pat. Nos. 4,857,630 and 5,196,502. In such a process, monomers with pendent groups capable of supporting polymer chain growth are added to the polymerization reaction along with the other monomers. The degree of branching in this reaction is proportional to the relative concentration of the reactive monomer. One of the drawbacks of this method is that the chains produced are of varying lengths and these reactions have to be closely monitored and controlled due to their tendency to form highly cross-linked and insoluble gels. A further drawback is that the entire dendritic polymer will be composed of the same polymer or copolymer. In certain applications, it is desirable to have the inner branch polymers and the outer branch polymers to differ in composition and type.
Another reaction method, in which dendrimers are produced, involves incorporating into the growing polymer chain, xe2x80x9cblockedxe2x80x9d monomers with pendent groups capable of supporting polymer chain growth. The xe2x80x9cblockingxe2x80x9d prevents the monomers from binding to other groups and therefore prevents the generation of sub-branches. Following the synthesis of the linear polymer, these sites can be de-blocked and reacted with monomers to generate the required branching. Examples of this method are provided in U.S. Pat. Nos. 4,853,436 and 6,084,030. This method avoids the problem of cross-linking, and allows for the generation of branched polymers which have different chains on the inner and outer surface. The drawback of this method is that a new reaction is required to change the degree of branching. Furthermore, reaction conditions need to be optimized to achieve the exact mixture of reactive and non-reactive monomers in a chain, and the length of the chain. Also, the nature and characteristics of the polymers may change due to the addition of the reactive monomers in the polymer chain.
The third method for generating dendritic polymers is by graft polymerization. Examples of this process are provided in U.S. Pat. Nos. 5,397,841 and 5,631,329. According to this method, a polymer is generated incorporating reactive monomers with pendent groups capable of accepting an addition reaction. A second set of polymers, which will become the branches, are generated with functionalized end groups. Under appropriate conditions, the functionalized end groups form chemical bonds with the activated pendent groups. The newly formed branches may also have incorporated within them pendent groups which are capable of supporting further branch formation. This method avoids the problem of cross-linking, and allows for the generation of branched polymers which have different chains on the inner and outer surface. It overcomes the above mentioned drawback of uniform chain length since the chains are preformed and can be characterized prior to incorporation. However, the drawback with this method is that a different synthesis reaction is required to change the degree of branching. Furthermore, the reaction conditions need to be optimized to achieve the exact mixture of reactive and non-reactive monomers in a chain. In addition, the nature and characteristics of the polymers may change due to the addition of reactive monomers in the polymer chain.
The final method for generating dendritic polymers by graft polymerization is by introducing chloromethyl groups onto linear polymer, and grafting polymers onto those reactive groups, as described in U.S. Pat. No. 5,631,329. The chloromethyl groups are capable of undergoing substitution reactions with the functionalized end groups on other polymers. The newly formed branched polymer may also be provided with further chloromethyl reactive groups thereby allowing for dendritic branching of the polymer. This method has the advantage that the degree of branching is controlled by the number of chloromethyl groups added. This method also has the advantage that additional polymerization steps are not required. Linear polymers of defined length and composition can be used. The disadvantage of this method is that a large excess of chloromethyl methyl ether, a potent carcinogen, is required to generate the chloromethyl grafting sites. A further limitation is the requirement that the optimum temperature for the reaction is xe2x88x9230xc2x0 C.
It is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.
According to a preferred embodiment of the present invention, there is provided a method for the synthesis of branched or dendritic polymers, which may be either homopolymers of copolymers. The process involves a first polymer which is acylated, and a second polymer which is grafted onto the acyl groups. The first and second polymer may be linear, branched or dendritic.
In accordance with this aspect of the present invention, there is provided a process wherein a polymer may be subjected to at least two cycles of acylation and grafting.
The present invention provides, in another aspect, the polymers produced by said process, which may be in the form of simple branched (comb polymers), or dendritic polymers.